Semiconductor device and method of manufacturing therefor

ABSTRACT

An active region on a semiconductor substrate is electrically isolated by trench isolation. A structure of the trench isolation is constituted of a trench; a silicon oxide film formed on the inner wall of trench; an oxidation preventive film formed between silicon oxide film and semiconductor substrate; and a filling oxide film filling trench. Gate oxide film is formed by oxidation having a high capability by which radicals of at least one kind of hydrogen radicals and oxygen radicals are generated. Thereby, gate oxide film is formed so as to have a almost uniform thickness such that a thickness of a region directly above oxidation preventive film and a thickness of a region directly below gate electrode are almost the same is each other. According to the above procedure, there are obtained a semiconductor device having good transistor characteristics and a fabrication process therefor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and afabrication process therefor, and more particularly, to a semiconductordevice forming an oxidation preventive film in a trench isolationstructure therein and a fabrication process therefor.

2. Description of the Background Art

A stress is produced in the inner wall of a trench due to volumeexpansion of the inner wall of the trench, caused by oxidation in anoxidation process after formation of a trench isolation structure tothereby generate crystal defects such as dislocations and micro-defectsin a silicon (Si) substrate. In order to prevent generation of thecrystal defects, there has been available a technique forming anoxidation preventive film on the inner wall of the trench. Descriptionwill be given of a prior art technique forming the oxidation preventivefilm below.

FIGS. 18 to 24 are schematic sectional views showing a sequence of stepsof a method of manufacturing a prior art semiconductor device. Referringto FIG. 18, for example, an insulating film 102 is formed on a p typesilicon substrate 101.

Referring to FIG. 19, insulating film 102 is patterned by aphotolithographic technique and an etching technique at the ordinarylevels. By use of any convenient etching technique such as anisotropicdry etching with the patterned insulating film 102 as a mask, a trench103 of a prescribed depth is formed on a surface of silicon substrate101.

Referring to FIG. 20, in order to remove a damaged layer caused by theetching and furthermore, rounding the top edge portion of trench 103, asilicon oxide layer 104 is formed on the inner wall of trench 103.

Referring to FIG. 21, in order to prevent oxidation of the inner wall oftrench 103 in a subsequent oxidation step, an oxidation preventive film106 is formed. Oxidation preventive film 106 is formed as a siliconnitride film at the interface between silicon substrate 101 and siliconoxide film 104 by annealing silicon substrate 101 in an atmosphereincluding nitrogen (N).

Referring to FIG. 22, a filling oxide film 107 constituted of a siliconoxide film is formed on insulating film 102 so as to fill trench 103.Thereafter, by annealing silicon substrate 101 at a prescribedtemperature in a prescribed atmosphere, filling oxide film 107 isdensified. Thereafter, the surface of silicon substrate 101 isplanarized by means of a CMP (Chemical Mechanical Polishing) method andsubsequently, insulating film 102 on an active region is removed by wetetching.

Referring to FIG. 23, the CMP and the wet etching exposes the surface ofsilicon substrate 101 while leaving filling oxide film 107 so as to filltrench 103, thus completing a trench isolation structure.

Referring to FIG. 24, a gate oxide film 108 is formed on the surface ofsilicon substrate 101 by oxidation. The oxidation is effected byintroducing hydrogen gas and oxygen gas into a reaction vesselaccommodating wafers after the gases react with each other, orintroducing only oxygen gas into the reaction vessel. Thereafter, a gateelectrode is formed on gate oxide film 108 and subsequent to this, animpurity is ion implanted into silicon substrate 101 with the gateelectrode or the like as a mask, thereby forming a pair of source/drainregions on the surface of silicon substrate 101. In such a way, thereare formed a MOS (Metal Oxide Semiconductor) transistor used in DRAM(Dynamic Random Access Memory) and others, and a floating-gatetransistor used in EEPROM (Electrically Erasable Programmable Read OnlyMemory) and others.

In the above semiconductor device, oxidation preventive film 106 isformed on the inner wall of a trench isolation structure. Therefore, afilm thickness of gate oxide film 108 shows thinning as depicted in FIG.25 at the top edge portion of the trench isolation structure (on theoxidation preventive film 106). That is, a film thickness TA3 of gateoxide film 108 at the top edge portion of the trench isolation structureis thinner than those of the other parts, having resulted in a problemof difficulty in forming a high reliability gate oxide film 108.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a semiconductordevice capable of suppressing thinning of a gate oxide film at the topedge portion of a trench isolation structure therein and a method ofmanufacturing therefor.

A method of manufacturing a semiconductor device of the presentinvention includes the following steps.

First of all, a trench is formed on a main surface of a semiconductorsubstrate. Then, an oxidation preventive film is formed along the innerwall of the trench. A filling layer is formed so as to fill the trench.A high oxidation capability is applied on the main surface of asemiconductor substrate in an atmosphere in which radicals of at leastone kind of hydrogen radicals and oxygen radicals are generated tothereby form a gate oxide film on the main surface of a semiconductorsubstrate.

In a method of manufacturing a semiconductor device of the presentinvention, since an oxidation with a high capability is applied by whichradicals of at least one kind of hydrogen radicals and oxygen radicalsare generated, oxidation speeds of the semiconductor substrate and theoxidation preventive film can be almost the same as that of each other.Therefore, a thickness of a gate oxide film, formed by this oxidation,directly above the oxidation preventive film at the top edge portion ofthe step in the trench can be of the same order as those of the gateoxide film in the other regions. With this, the film thickness of thegate oxide film can be uniform to thereby obtain a high reliability gateoxide film.

The above method of manufacturing a semiconductor device preferablyfurther includes: a step of forming a gate electrode on the gate oxidefilm; and a step of forming a pair of source/drain regions on the mainsurface of a semiconductor substrate so as to sandwich a region directlybelow the gate electrode between the source/drain regions.

With such steps added, a transistor with a gate layer can be formed.

In the above method of manufacturing a semiconductor device, the gateelectrode is preferably formed so as to have a floating gate and acontrol gate, insulated from each other.

Thereby, a memory cell of a flash memory can be fabricated.

In the above method of manufacturing a semiconductor device, the gateoxide film preferably has almost the same thickness in a region directlyabove the oxidation preventive film and a region directly below the gateelectrode.

In such a way, the gate oxide film having a uniform thickness can beformed.

In the above method of manufacturing a semiconductor device, theoxidation preventive film is preferably made from at least one of asilicon nitride film and a silicon oxynitride film.

In such a way, various kinds of films can be chosen as an oxidationpreventive film.

A semiconductor device of the present invention includes: asemiconductor substrate; an oxidation preventive film; a filling layer;a gate oxide film; and a gate electrode. The semiconductor substrate hasa trench on a main surface of the semiconductor substrate. The oxidationpreventive film is formed along the inner wall of the trench. Thefilling layer fills the trench. The gate oxide film is formed on themain surface of the semiconductor substrate and the oxidation preventivefilm. The gate electrode is formed on the gate oxide film. The gateoxide film has almost the same thickness in a region directly above theoxidation preventive film and a region directly below the gateelectrode.

In a semiconductor device of the present invention, since the gate oxidefilm has almost the same thickness in a region directly above theoxidation preventive film and a region directly below the gateelectrode, a thickness of the gate oxide film can be uniform. Thereby, ahigh reliability gate oxide film can be obtained.

In the above semiconductor device, the gate electrode preferably has afloating gate electrode and a control gate, insulated from each other.

Thereby, a memory cell of a flash memory can be fabricated.

In the above semiconductor device, the oxidation preventive film ispreferably made from at least one of a silicon nitride film and asilicon oxynitride film.

In such a way, various kinds of films can be selected as an oxidationpreventive film.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 7 are schematic sectional views showing a sequence of stepsof a method of manufacturing a semiconductor device in a firstembodiment of the present invention;

FIG. 8 is a schematic sectional view showing a structure of a MOStransistor formed after a gate oxide film of the semiconductor device inthe first embodiment of the present invention is formed;

FIG. 9 is a schematic sectional view for describing a film thickness ofthe gate oxide film of the semiconductor device of the first embodimentof the present invention;

FIG. 10 is a schematic sectional view showing indispensable featurescombined of the first embodiment of the present invention, which isapplied to a floating gate transistor;

FIGS. 11 to 14 are schematic sectional views showing a sequence of stepsof a method of manufacturing a semiconductor device in a secondembodiment of the present invention;

FIG. 15 is a schematic sectional view showing a structure of a MOStransistor formed after a gate oxide film of the semiconductor device inthe second embodiment of the present invention is formed;

FIG. 16 is a schematic sectional view for describing a film thickness ofthe gate oxide film of the semiconductor device of the second embodimentof the present invention;

FIG. 17 is a schematic sectional view showing indispensable featurescombined of the second embodiment of the present invention, which isapplied to a floating gate transistor;

FIGS. 18 to 24 are schematic sectional views showing a sequence of stepsof a method of manufacturing a prior art semiconductor device; and

FIG. 25 is a schematic sectional view for describing a film thickness ofa gate oxide film of the prior art semiconductor device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Description will be given of embodiments of the present invention belowon the basis of the accompanying drawings.

First Embodiment

First of all, referring to FIG. 1, an insulating film 2 is formed on asemiconductor substrate 1 made of silicon of a p conductivity type, forexample.

Referring to FIG. 2, insulating film 2 is patterned by aphotolithographic technique and an etching technique at the ordinarylevels. By use of any convenient etching technique such as anisotropicdry etching with patterned insulating film 2 as a mask, a trench 3 of adesired depth is formed on a surface of semiconductor substrate 1.

Referring to FIG. 3, in order to remove a damaged layer caused by theetching and further, round the top edge portion of trench 3, thermaloxidation is applied onto semiconductor substrate 1. By doing so, asilicon oxide film 4 is formed on the inner wall of trench 3 to athickness of from 10 nm to 70 nm.

Referring to FIG. 4, in order to prevent oxidation of the inner wall oftrench 3 that would be caused by a subsequent oxidation step, annealingis applied to semiconductor substrate 1 in an atmosphere including atleast one of NO gas, N₂O gas and NH₃ gas at a temperature in the rangeof from 850° C. to 1000° C. With such annealing, an oxidation preventivefilm 6 made of a silicon nitride film is formed at the interface betweensemiconductor substrate 1 and silicon oxide film 4.

Referring to FIG. 5, a silicon oxide film such as a TEOS (Tetra EthylOrtho Silicate) oxide film, a HDP (High Density Plasma) oxide film orthe like is formed on insulating film 2 so as to fill the interior oftrench 3 by means of a LPCVD (Low Pressure Chemical Vapor Deposition)method. Thereafter, annealing is performed on semiconductor substrate 1at 800° C. to 1150° C. in a N2 (nitrogen) atmosphere for densificationof a filling oxide film 7. After filling oxide film 7 is removed by CMPto planarize the surface of semiconductor substrate 1, insulating film 2on an active region is removed by wet etching.

Referring to FIG. 6, the surface of semiconductor substrate 1 is exposedby the above CMP and the wet etching to leave filling oxide film 7 onlyin trench 3 and complete trench isolation.

Referring to FIG. 7, a gate oxide film 8 is formed on the active regionof semiconductor substrate 1. Gate oxide film 8 is formed under aso-called steam condition, that is under an oxidative condition in whichhydrogen radials and oxygen radicals are generated, and having so highan oxidative capability that oxidation preventive film 6 such as asilicon nitride film can be oxidized. To be concrete, hydrogen gas andoxygen gas are separately introduced into a reaction vesselaccommodating wafers to react with each other directly above the wafersand thereby generate hydrogen radicals and oxygen radicals, whichgenerates oxidation with a high capability. In such a manner, there areformed a trench isolation structure and gate oxide film 8 in theembodiment.

Gate oxide film 8 thus formed can be used as a gate insulating film ofan ordinary MOS transistor as shown in FIG. 8, for example. Such a MOStransistor is formed in a way described below after the step of FIG. 7.

Referring to FIG. 8, a conductive layer used in a gate electrode isformed on gate oxide film 8 and thereafter, patterned by aphotolithographic technique and an etching technique at the ordinarylevels to form a gate electrode 9. An n type impurity such as arsenic orphosphorus is ion implanted in an active region of semiconductorsubstrate 1 with gate electrode 9 as a mask. With the ion implantationapplied, a pair of source/drain regions 10 are formed on the surface ofsemiconductor substrate 1 so as to sandwich a region directly below gateelectrode 9 between source/drain regions 10 to thereby complete a MOStransistor.

Then, description will be given of a structure of the semiconductordevice fabricated as described above.

Referring to FIG. 8, the active region of semiconductor substrate 1 iselectrically isolated by trench isolation. The trench isolationstructure is constituted of silicon oxide film 4 formed along the innerwall of trench 3 provided on semiconductor substrate 1; oxidationpreventive film 6 made of a silicon nitride film or the like formed atthe interface between silicon oxide film 4 and semiconductor substrate1; and filling oxide film 7 filling trench 3.

A MOS transistor is formed in the active region electrically isolated.The MOS transistor has: gate oxide film 8; gate electrode 9: and pair ofsource/drain regions 10. Gate oxide film 8 is formed on the activeregion of semiconductor substrate 1 and gate electrode 9 obtained bypatterning is formed on gate oxide film 8. Pair of source/drain regions10 is formed on the surface of semiconductor substrate 1 so as tosandwich the region directly below gate electrode 9 between source/drainregions 10.

Gate oxide film 8 described above has a uniform thickness such that athickness TA1 of a region directly above oxidation preventive film 6 anda thickness TB1 of a region directly below gate electrode 9 are equal toeach other as shown in FIG. 9.

In the embodiment, an oxidizing method is applied, in which hydrogenradicals and oxygen radicals are generated, and which has so high anoxidative capability that oxidation preventive film 6 made of a siliconnitride or the like can be oxidized, thereby forming gate oxide film 8.For this reason, in the oxidation, oxidizing speeds on oxidationpreventive film 6 made of a silicon nitride and semiconductor substrate1 made of silicon can be the same as each other. Thereby, gate oxidefilm 8 as shown in FIG. 9 comes to have almost the same thickness in aregion directly above oxidation preventive film 6 and in a regiondirectly below gate electrode 9, thereby enabling prevention of thinningof gate oxide film 8 at the top edge portion of the trench isolationstructure.

Since gate oxide film 8 has a uniform thickness in such a way, gateoxide film 8 has difficulty in deterioration and increases a breakdownlifetime, thereby enabling a high reliability gate oxide film 8 to beachieved. Therefore, a device having good transistor characteristics canbe obtained that is categorized in DRAM (Dynamic Random Access Memory)or the like.

Not that after the step shown in FIG. 7, a floating gate transistorshown in FIG. 10 can also be formed, which will be described below.

Referring to FIG. 10, after a floating electrode 9 a is formed on gateoxide film 8, an insulating film 9 b and a control gate electrode 9 care formed on floating gate electrode 9 a. An n type impurity such asarsenic or phosphorus is ion implanted with control gate electrode 9 cor the like as a mask. By doing so, pair of source/drain regions 10 isformed on the surface of semiconductor substrate 1 to thereby completethe floating gate transistor.

The floating gate transistor thus fabricated has floating gate electrode9 a and control gate electrode 9 c, insulated from each other. Since astructure of the floating gate transistor is almost the same as that ofthe above ordinary MOS transistor of FIG. 8, the same symbols areattached to the same constituents and description thereof is omitted.

In the above floating gate transistor, too, by preventing thinning ofgate oxide film 8 at the top edge portion of the trench isolationstructure, gate oxide film 8 having a uniform thickness can be obtained.By doing so, gate oxide film 8 has difficulty in deterioration and along breakdown lifetime, thereby enabling a flash memory having goodtransistor characteristics to be obtained.

Second Embodiment

A fabrication process of the embodiment is different from that of thefirst embodiment by comparison in a step of forming an oxidationpreventive film. A fabrication process of the embodiment follows asequence of steps similar to that of the first embodiment shown in FIGS.1 to 3. Thereafter, there is formed an oxidation preventive film 5constituted of a silicon nitride film (an SiN film) and a siliconoxynitride (an SiON film) to a thickness from 5 nm to 30 nm.

Referring to FIG. 12, a filling oxide film 7 is formed in a similar wayto that of the first embodiment 1 so as to fill trench 3. Thereafter,the surface of silicon substrate 1 is planarized by CMP, followed byremoval of insulating film 2 on the active region with wet etching.

Referring to FIG. 13, by removal of insulating film 2, the surface ofsemiconductor substrate 1 is exposed, leaving filling oxide film 7 intrench 3 only, to complete trench isolation.

Referring to FIG. 14, an gate oxide film 8 is formed in conditionssimilar to those of the first embodiment. In formation of gate oxidefilm 8, an oxidation method with a high capability is employed;therefore, oxidation preventive film 5 is also oxidized to form gateoxide film 8 in a region directly above oxidation preventive film 5. Insuch a way, the trench isolation structure and gate oxide film 8 in theembodiment are formed.

Gate oxide film 8 thus formed can be used as a gate insulating film ofan ordinary MOS transistor as shown in FIG. 15, for example. Such a MOStransistor is formed after the step of FIG. 14, similar to the firstembodiment.

Then, description will be given of a structure of the semiconductordevice fabricated as described above.

Referring to FIG. 15, the active region of semiconductor substrate 1 iselectrically isolated by trench isolation. The trench isolationstructure is constituted of silicon oxide film 4 formed along the innerwall of trench 3 formed on semiconductor substrate 1; oxidationpreventive film 5 formed along the inner wall of silicon oxide film 4;and filling oxide film 7 filling trench 3.

A MOS transistor is formed in the active region electrically isolated bythe trench isolation. The MOS transistor has: gate oxide film 8; gateelectrode 9; and pair of source/drain regions 10. Gate oxide film 8 isformed on the active region of semiconductor substrate 1 and gateelectrode 9 obtained by patterning is formed on gate oxide film 8. Pairof source/drain regions 10 are formed on the surface of semiconductorsubstrate 1 so as to sandwich a region directly below gate electrode 9between source/drain regions 10.

Gate oxide film 8 described above has a uniform thickness such that athickness TA2 of a region directly above oxidation preventive film 5 anda thickness TB2 of a region directly below gate electrode 9 are equal toeach other as shown in FIG. 16.

In the embodiment as well, similar to the first embodiment, thinning ofgate oxide film 8 at the top edge portion of the trench isolationstructure can be prevented from being produced and gate oxide film 8 hasa uniform film thickness. For this reason, gate oxide film 8 hasdifficulty in deterioration and increases a breakdown lifetime, therebyenabling a high reliability gate oxide film 8 to be attained. Therefore,a device having good transistor characteristics can be obtained that iscategorized in DRAM or the like.

Furthermore, while in FIG. 15, description is given of a structure of anordinary MOS transistor, a fabrication process and structure of theembodiment, as shown in FIG. 17, can be applied to a floating gatetransistor as well. In this case, gate oxide film 8 has difficulty indeterioration and a long breakdown life time, thereby enabling a flashmemory with good transistor characteristics to be obtained.

Note that conductivity types associated with constituents of the abovesemiconductor device may be all inverted in polarity.

Furthermore, conditions for oxidation with a high capability in gateoxide film formation are not limited to the above conditions, but anycondition may be adopted as far as an oxidation preventive film such asa silicon nitride film can be oxidized at almost the same speed as issilicon.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1-8. (canceled)
 9. A semiconductor device comprising: a semiconductorsubstrate having a trench at a main surface thereof; an oxidationpreventive film including a nitrogen formed along an inner wall; afilling layer filling said trench with said preventive film locatedbetween said filling layer and said semiconductor substrate; a gateoxide film formed on said main surface of said semiconductor substrateand said oxidation preventive film; and a gate electrode formed on saidgate oxide film, wherein said gate oxide film has substantially the samethickness in a region directly above said oxidation preventive film anda region directly below said gate electrode between source and drainregions, and said gate oxide film directly above said oxidationpreventive film includes said nitrogen.
 10. The semiconductor deviceaccording to claim 9, wherein said gate electrode has a floating gateelectrode and a control gate, insulated from each other.
 11. Thesemiconductor device according to claim 9, wherein said oxidationpreventive film is made from at least one of a silicon nitride film anda silicon oxynitride film.